The development of electrical apparatus such as electrical machines for use in high temperature environments places significant demands on components associated with the apparatus. In particular, there is a requirement for stability of the materials from which the components are constructed. Such machines generally comprise electrical windings embedded in an electrical encapsulant, sometimes also known as an impregnant. Such encapsulants provide mechanical stability and secondary electrical insulation of the electrical windings in addition to that provided by the insulating material surrounding the wires of the coil. Environments requiring stability of electrical encapsulants at high temperature include those associated with nuclear reactors and next generation aircraft motors and generators, particularly where forced cooling is not available.
In addition to heat from an environment in which a component is situated, a component may be subjected to heat due to other factors such as an electrical current carried by a conductor as well as other stresses. For example, encapsulant compositions used to mechanically encapsulate electrical windings of high temperature electrical machines are subject to particularly harsh thermal and mechanical conditions. The integrity of such encapsulants is critical to continued successful operation of the motor or generator. In some cases, encapsulants in electrical machines must operate for 20 years or longer at temperatures of 450° C. or higher.
A major barrier restricting the operating temperature of electrical machines is the limited thermal stability of the encapsulant material. Breakdown of encapsulant materials can occur at excessively high temperatures, or following prolonged exposure to high temperatures. Such breakdown may lead to mechanical or electrical failure of the electrical machine. High temperature electrical encapsulants must also be relatively flexible in order to accommodate movement and thermal expansion of the encapsulated coils.
Such encapsulants must also be compatible with wire insulation used in high temperature electrical machines. By “compatible”, it will be understood that the encapsulant and wire insulation must have similar physical and chemical properties, such as having similar coefficients of thermal expansion, and being relatively chemically inert with each other at typical operational temperatures.
Suitable high temperature wire insulation materials generally comprise solid or woven ceramic based materials. One known high temperature wire insulation material is described in international patent application WO2009150463. The wire insulation material described therein comprises a first organo-alkoxide 1RxSi(O1R′)4-x and a second organo-alkoxide 2RxSi(O2R′)4-x, where 1R is a non-hydrolysable organic moiety thermally stable to a temperature of at least 150° C., 2R is a non-hydrolysable organic moiety containing a functional group that can react with another like functional group to form an organic polymer, 1R′ and 2R′ are alkyl radicals and x is an integer from 0 to 3; and an inorganic filler material which together form an inorganic-organic nano hybrid material.
Inorganic-organic nano hybrid materials are materials comprising organic and inorganic components linked together by covalent or non-covalent bonds at a nano-meter scale. These materials differ from nano-composites for example by the chemical bonding between the organic and inorganic components. Examples and a discussion of prior nano-hybrid materials is disclosed in “Definitions and Categories of Hybrid Materials” by Makoto Nanko, published in The AZo Journal Of Materials Online.
The present invention describes an electrical encapsulant composition and an electrical machine comprising the electrical encapsulant which seeks to overcome some or all of the above problems.